Pulverizing of coal



A ril 3, 1934. E CAREY ET AL 1,953,847

PULVERI ZING OF COAL Filed Aug. 24, 1953 2 Sheets-Sheet l I N VEN TORS [Wind/2am fi'mzqis Carey jiHgar/l'nnk Bran Ii W k )M ATTORNEYS April 3, 1934= w. F. CAREY ET AL PULVERIZ ING OF COAL Filed Aug. 24, 1933 2 Sheets-Sheet 2 IN VEN TORS Wi'ml/mm/ flancl'z; (21 B Y 515701 fizz/2k brawn W )M M/A TTORNEYS.

Patented Apr 3, 1934 nirsn STATES PATENT QFFEE N orton-on-Tees, England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain Application August, 24,

1933, Serial No. 686,640

- In Great Britain August 13, 1932 5 Claims.

This invention relates to the pulverizing of coal and has for its object the reduction of power necessary in the pulverizing operation andthe reduction of weartaking place on the pulverizing surfaces.

In pulverizing coal to a given average particle size by any of the methods commonly used in the art, it is always found that only a proportion of the product, say 70 per cent, lies within the range of particle size required and of the remainder some has escaped pulverization and. some has been pulverized to a very much finer state of division. The production of these fines is' largely responsible for the heavy cost of pulverizing, since on the one hand the energy required in their production is out of all proportion to that required for producing the material of the desired particle size, and on the other, their removal from the product necessitates a costly system of air classification or screening.

We havenow found that coal can be pulverized economically to predetermined limits of particle size; lying below one twenty-fifth of an inch, in a multi-stage roll crusher constructed as hereinafter described. We have also found that there is a certain critical stage reduction lying between 3.1 and 3.6 according to the kind of coal, above which wear of the pulV-erizing surfaces increases very rapidly and below which the amount of wear is very nearly independent of the stage reduction. By stage reduction is meant the ratio of the average particle size before and after passage of the material through a particular stage. The stage reduction ratio is also equal to the ratio between the gaps in successive stages. At and below the critical stage reduction ratio there is a diminished tendency of the coal to agglomerate into flakes which, if allowed to pass into the next set of rolls produce very high local stresses on the roll surface, leading to indentation and excessive wear on the roll surfaces. Experiments with vario-us sorts of coal have shown that the amount of flakes increases by about ten times if the reduction ratio be increased beyond the critical value. The flakes can, of course, be removed between stages by the interposition of suitable screens, but it is none the less desirable to minimize their production, as the flaked material represents a waste as far as the pulverizlng process is concerned.

According to the present invention therefore, coal is pulverized to particles less than one twen ty-flfth of an inch in size, in a multi-stage roll crusher in which the individual stage reduction ratios, with the possible exception of the ratio for the last set of rolls, lie between 2.0 and 3.6, or in which the average stage reduction ratio lies between these limits. The reduction ratio for the last pair of rolls may be greater than 3.6, since flakes formed therein have no further opportunity of causing wear. The limits of 2.0 and 3.6 are chosen because ratios greater than 3.6 cause rapid wear as indicated above and ratios less than 2.0 will entail an expensive mill and a somewhat heavy power consumption owing to the greater number of moving parts. Preferably, to remove flakes, the coal is screened between two or more successive stages. The invention is applicable chiefly to a maximum initial particle size of a My, because above this size the coalcan sometimes be more economically crushed in gyratory crushers.

For convenience of construction and the advantage of interchangeability we prefer to use a machine having an equal width of roll face at each stage. In order to ensure that, as far as possible, the particles are presented singly and separately to the rolls, the initial feed should be arranged so that at least per cent. or" free space is left in the gap between the first pair of rolls. The feed should be screened to eliminate tramp iron (nuts, etc. accidentally detached from the machinery used in the previous handling of the coal). The left and right hand rollers of each pair are driven clockwise and counter-clockwise respectively and at the same peripheral speed. The peripheral speed of successive pairs of rolls should increase in a downward direction in at least a ratio inverse to the stage reduction ratio, in order to avoid choking, and accordingly the last pair of rolls will have the maximum peripheral speed which is governed by the allowable stress in the material used for constructing the rolls. The velocity of the coal falling from gap to gap is not determined by gravity alone, but also by the air current which is entrained by the rolls. This ensures that there is practically no relative velocity between the coal and the rolls surfaces so that a pure crushing action results. The rolls are preferably made of uniformly hard material such as nickelchrome steel, or are surfaced with a substantial thickness of such material. superficially hardened rolls such as chilled cast iron or case-hard- 105 ened steel are not so satisfactory.

The screens used for separating flakes between the stages should have a mesh about two to three times as wide as the gap between the preceding rolls. Such screens ofier an easy passage to the no 'lop 2nd 3rd Bottom rolls rolls rolls rolls Reduction ratio 3.16 3.16 2.0 5.0 Width of gap l/l3 U40 U l/400 Surfacespeed ofrollsft./soc 3.16 10 20 100 R. P. M. for 10 rolls 72. 5 229 458 2290 A machine for 50:1 total reduction ratios would be similar but with an average stage reduction ratio of 2.66, whilst a machine for 200:1 total reduction ratio would be a five-stage machine with an average reduction of 2.88:1, the roll speeds in both cases being increased from stage to stage in proportion to the reduction ratios used. In the case of the five-stage machine a screen would be placed between the top and second rolls and between the third and fourth rolls, possibly also over the top rolls.

A machine according to the invention is illustrated in the accompanying drawings in which Figure 1 is an end elevation, Figure 2 a front elevation, partly in section, and Figure 3 a sectional view on the line 3-3 of Fig. 2. The machine is driven by means of a motor coupled to the shaft 1 and motion is transmitted to the various rolls through one of each pair of rolls 2 by means of the gearing 3. The other member of each pair of rolls is driven in the opposite direction and at the same speed by means of the equal gearing 4. The coal which is to be pulverized is fed into the bunker 5 and passes over a magnetic pulley 9 before going to the rolls. This pulley removes tramp iron and delivers it into the chute 10 from which it passes out at the point 11. The pulverized coal leaves the machine at the outlet branch 6 and removal from the machine may be assisted by means of a screw conveyor 7. A screen 8 is fitted between the 2nd and 3rd pairs of rolls and is vibrated by means of cams deriving motion from the main driving gear. The screen is inclined to the horizontal and delivers separated material into the chute 12 and leaves the machine at the point 13. The distance between the rolls may be adjusted by means of the screws 14.

We are aware that roll crushers have previously been employed for crushing large pieces of coal down to pieces of about A" in size, but the pulverizing of coal to particles less than one twenty-fifth of an inch in size presents quite a different problem, as, unless the special and hitherto unrecognized precautions hereinbefore referred to are observed, the wear on the rolls is so great that an accurate adjustment of the gap between the rolls is no longer possible and the rolls need re-surfacing or replacing so often that the process is uneconomic.

We claim:

1. A process for pulverizing coal which comprises crushing the coal of a particle size of about one-fourth of an inch in a multi-stage roll crusher in which the individual stage reduction ratios lie between 2.0 and 3.6.

2. A process for pulverizing coal which comprises crushing the coal of a particle size of about onefourth of an inch in a multi-stage roll crusher in which the average stage reduction ratio lies between 2.0 and 3.6.

3. A process for pulverizing coal to particles less than one-twenty-fifth of an inch in size which comprises crushing the coal in a multistage roll crusher in which the individual stage reduction ratios lie between 2.0 and 3.6.

4. A process for pulverizing coal to particles less than one-twenty-fifth of an inch in size which comprises crushing the coal in a multistage roll crusher in which the average stage reduction ratio lies between 2.0 and 3.6.

5. A process as claimed in claim 3 in which the coal is screened between two or more successive stages to remove flakes.

EDGAR FRANK BRAWN. WINDHAM FRANCIS CAREY. 

